The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities.
It relates more particularly to implantable devices to continuously monitor the heart rhythm and deliver if necessary electrical pulses to the heart to ensure joint and permanent stimulation of the left and right ventricles to resynchronize them, a technique called “CRT” (Cardiac Resynchronization Therapy) or “BVP” (Bi-Ventricular Pacing).
A CRT pacemaker is disclosed, for example, in EP 1108446 A1 (ELA Medical), which describes a device for applying, between the respective moments of stimulation of the left and right ventricles, a variable delay called “interventricular delay” (DVV or VVD), adjusted to resynchronize the contraction of the ventricles with fine optimization of the patient's hemodynamic status. Indeed, a simultaneous stimulation of both ventricles is not always optimal. It is not always optimal because it does not necessarily result in a synchronous contraction of both ventricles. Conduction delays in the myocardium that are not the same in the right and in the left cavities can be present, and can depend on multiple factors. Furthermore, depending on the location of the left ventricular lead, whether it is introduced into the coronary sinus or it is an epicardial lead.
In some cases it is desirable to establish a delay between the two stimuli, and to adjust this delay to resynchronize the contraction of the ventricles and thus ensure a fine optimization of hemodynamics. The VVD may be zero, positive (the left ventricle is stimulated after the right ventricle) or negative (the right ventricle is stimulated after the left ventricle).
CRT devices can include a classic “dual chamber” mode wherein the device monitors ventricular activity after a spontaneous (detection of a P wave of atrial depolarization) or stimulated (application of an atrial stimulation A pulse) atrial event. At the same time, the device starts counting a delay called “atrioventricular delay” (DAV or AVD) such that if no spontaneous ventricular activity (R wave) has been detected at the end of this period, then the device initiates stimulation in this ventricle (V pulse application). Often dramatic results have been observed in clinical studies for patients with heart failure not improved by conventional treatments, since the parameters of the CRT therapy are accurately adjusted according to the patient and the nature of his myocardial contraction condition (e.g., such as dilatation of the heart chambers, low ejection fraction, and excessive lengthening of the QRS complex—whether this disorder is spontaneous or induced by traditional stimulation).
Various clinical studies have shown that the endocardial acceleration (hereinafter “EA”) is a parameter that relatively precisely and in real time reflects the phenomena related to movements of the heart chamber. An EA signal can provides comprehensive information on cardiac mechanics, both for normal operation and for a deficient functioning. The endocardial acceleration is measured, for example, by an accelerometer integrated into an endocardial lead, for example as described in EP 0515319 A1 (Sorin Biomedica Cardio SpA).
EP 1736203 A1 (ELA Medical) describes a technique for evaluating in a simple, rapid, automated and accurate method the impact of various parameters of the CRT stimulation, including AVD and VVD delays, by running an AVD scan in a given stimulation configuration (that is to say, with a given DVV), to plot a characteristic giving the value of the peak of endocardial acceleration PEA versus the AVD, so as to select the optimal pair {AVD, VVD}. The intent is to determine, at the time of implantation or later, the optimal pacing configuration that optimizes the patient's hemodynamic status.
However, effective therapy involves the delivery of appropriate stimulation pulses that provide ventricular capture (that is to say, the applied pulse has induced, both in the right and left cavities, a depolarization of the cavity), as it is an essential condition for the resynchronization. To verify the presence of this capture, devices are provided with “capture test” methods for determining whether a stimulation has been effective or not for one or both of the ventricles. These methods operate by search and analysis of the “evoked wave” (the depolarization wave induced by stimulation of the concerned cavity).
The capture test can provide for an assessment of the “triggering threshold.” The triggering threshold is the minimum pacing energy required to cause depolarization of the ventricle (the thresholds may be different between the right site and the left site). It may be desirable to adjust the amplitude and/or the width of the stimulation pulse in order to guarantee that any stimulation cause an evoked wave and to ensure the energy delivered will not be excessive (e.g., so as not to affect too much the battery lifespan of the implant due to unnecessarily high energy consumption). The capture test can be performed at regular intervals, and can performed on a continuous, cycle-to-cycle basis.
A recent trend in biventricular stimulation is the multiplication of the “stimulation vectors.” This technique can involve applying the stimulation between different pairs of electrodes selected to optimize the applied therapy, each of the right and left leads being provided with a plurality of selectively switchable electrodes. Conventional “bipolar” stimulation is provided between a tip electrode (tip) and an annular electrode (ring) located near the right lead, or even on the left lead.
Some CRT devices also include an alternative called “pseudo-bipolar” configuration, wherein the stimulation of the left ventricle is caused by the electrode end (tip) of the left ventricular lead and the annular electrode (ring) or a defibrillation coil of the right ventricular lead. This “pseudo-bipolar” stimulation configuration of the left ventricle may provide benefits including reducing pacing thresholds and reducing the risk of stimulation of the phrenic nerve.
However, the use of multiple and different stimulation vectors can cause fluctuations of the associated pacing thresholds and create a corresponding increased risk of loss of capture. The use of multiple and different stimulation vectors can also create occurrence of a phenomenon called “anodal stimulation.” Anodal stimulation is characterized by a reversal of the anode and cathode during stimulation, with the result that the stimulation is not normally delivered (i.e. solely at the cathode) and the depolarization wave induced by this inverse stimulation may not be the desired one, and unexpected results can occur.
One example in the case of a “pseudo-bipolar” stimulation is as follows. Normally, the stimulation would occur between an electrode of the left lead (as cathode) and an electrode of the right lead (as anode) to stimulate the left ventricle, and the evoked wave is normally generated at the cathode (on the left ventricular site). But, in some situations, the depolarization wave of the left ventricle is not created only at the cathode (and therefore in the left ventricle), but also at the anode (and hence in the right ventricle). Therefore, the pulse to contract the left ventricle will also generate a right depolarization (“anodal stimulation” phenomenon). The consequence is that the depolarization wave corresponding to the pulse applied to the left cavity will produce a depolarization in the right ventricle at the same time. This can lead to loss of the benefit of an anticipated left stimulation during resynchronization therapy.
The risk of occurrence of anodal stimulation may be increased by the use of dedicated bipolar leads using a relatively small surface area of electrode as the anode. In pseudo-bipolar pacing, such an electrode can generate a high current density in its immediate environment, thus increasing the risk of triggering a sudden unexpected depolarization of the myocardium.
If anodal stimulation is not identified and corrected, this may remove the beneficial effects of a programmed VVD, and lead to sub-optimal resynchronization, characterized for example, by a concomitant contraction of both ventricles (a loss of VVD). This phenomenon is not rare in patients undergoing biventricular pacing. If it is detected, some devices may take reactive measures, such as changing the pacing configuration by switching to another stimulation vector not resulting in anodal stimulation (e.g. a pure bipolar vector, a unipolar vector or another interventricular vector by selection of other electrodes of the right and/or left leads).
Certain conditions favor the occurrence of a phenomenon of anodal stimulation. Such conditions may include: relatively high stimulation amplitudes, close to the stimulation threshold; and an “intercavity stimulation vector” or “pseudo-bipolar” configuration in which a stimulation is applied in a given cavity (e.g. the left ventricle) with the lead located on an anode in the opposite cavity (for example an electrode of a right ventricular lead).
These conditions can arise during the execution of a capture test, and thus repeatedly during a critical phase of the evaluation of the effectiveness of the stimulation. Indeed, the capture threshold test begins with high stimulation energy. This risk diminishes gradually with the reduction of the stimulation amplitude, but the test algorithm may have already been confused by anodal stimulation (that has not been identified as such) by a loss of capture. Such confusion can lead to misclassification of successive cycles of tests between capturing cycles and non-capturing cycles.
U.S. Pat. No. 6,687,545 B1 discloses a device for identification, during a capture test, of such a situation of anodal stimulation.